A mass spectrometry (MS) system in general includes an ion source for ionizing components of a sample under investigation, a mass analyzer for separating the gas-phase ions based on their differing mass-to-charge ratios (or m/z ratios, or more simply “masses”), an ion detector for counting the separated ions, and electronics for processing output signals from the ion detector as needed to produce a user-interpretable mass spectrum. Typically, the mass spectrum is a series of peaks indicative of the relative abundances of detected ions as a function of their m/z ratios. The mass spectrum may be utilized to determine the molecular structures of components of the sample, thereby enabling the sample to be qualitatively and quantitatively characterized. One popular type of MS is the time-of-flight mass spectrometer (TOF MS). A TOF MS utilizes a high-resolution mass analyzer (TOF analyzer). Ions may be transported from the ion source into the TOF entrance region through a series of ion guides, ion optics, and various types of ion processing devices. The TOF analyzer includes an ion accelerator that injects ions in packets (or pulses) into an electric field-free flight tube. In the flight tube, ions of differing masses travel at different velocities and thus separate (spread out) according to their differing masses, enabling mass resolution based on time-of-flight.
Ion mobility spectrometry (IMS) is a gas-phase ion separation technique in which ions produced from a sample in an ion source are separated based on their differing mobilities through a drift cell of known length that is filled with an inert gas of known composition and maintained at a known gas pressure and temperature. In low-electric field drift-type IM, the ions are urged forward through the drift cell under the influence of a relatively weak, uniform DC voltage gradient, for example in a range from 10 V/cm to 20 V/cm. The mobility of the ions depends largely on their collision cross-sections (CCSs) (and thus size and conformation or shape) and charge states (e.g., +1, +2, or +3), and to a much lesser extent their m/z ratios. Thus, ion separation by IM is largely orthogonal to ion separation by MS. From the drift cell the ions ultimately arrive at an ion detector, and the output signals from the ion detector are processed to generate peak information useful for distinguishing among the different analyte ion species detected. If the time that ions spent in the drift tube region is known and also the pressure and the voltage across the drift tube are known, then CCS can be calculated for any ion of interest. The CCS parameter is specific for the given molecule, instrument independent, and therefore can be utilized as a unique parameter for compound identification. Hence, the CCS parameter is of great interest in structural characterization of molecules and theoretical molecular dynamic simulations as well as in some other disciplines of science.
An IMS system may be coupled online with a mass analyzer, which often is a TOF analyzer. In the combined IM-MS system, ions are separated by mobility prior to being transmitted into the mass analyzer where they are then mass-resolved. Due to the significant degree of orthogonality between IM-based separation and MS-based separation, performing the two separation techniques in tandem is particularly useful in the analysis of complex chemical mixtures, including high-molecular weight (MW) biomolecules (biopolymers) such as polynucleotides, proteins, carbohydrates and the like. For example, the added dimension provided by the IM separation may help to separate ions that are different from each other (e.g., in shape) but present overlapping mass peaks. On the other hand, the added dimension provided by the MS separation may help to separate ions that have different masses but similar CCSs. This hybrid IM-MS separation technique may be further enhanced by coupling it with liquid chromatography (LC) or gas chromatography (GC) techniques. An IM-MS system is thus capable of acquiring multi-dimensional (IM-MS) data from a sample, characterized by acquisition time (i.e., chromatographic time or retention time), ion abundance (e.g., ion signal intensity), ion drift time through the IM drift cell, and m/z ratio as sorted by the MS.
An ion may activated through collision with a neutral gas molecule with a high enough collision energy to result in collisional heating, as opposed to collisional cooling, of the ion. With a high enough collision energy, ion activation can fragment the ion. This mechanism of ion fragmentation is typically implemented in a collision cell, and is referred to as collision-induced dissociation (CID) or collision-activated dissociation (CAD). Ion activation may also be utilized to cause a folded protein ion or other large biomolecular ion to unfold, which may be referred to as collision-induced unfolding (CIU). Ion activation followed by ion mobility separation is a powerful technique to identify closely related ions that can be difficult to identify using other techniques including ion mobility or mass spectrometry alone.
The hybrid IM-MS instruments currently available do not have an ion activation mechanism in the ion source that can achieve enough energy to unfold larger biomolecules or de-cluster larger biomolecules. Many commercial mass spectrometers can be equipped with a capillary-skimmer interface that couple the atmospheric pressure ionization region of the ion source with the first vacuum region in the mass spectrometer to allow moderate ion activation. Such a simple capillary-skimmer interface cannot provide high enough energy for collisional activation or fragmentation of larger bio-molecules. The typical pressure in a capillary-skimmer interface is less than 1 Torr. At higher pressures, this simple capillary-skimmer interface cannot provide high enough collision energy before electrical discharge. Therefore, larger bio-molecules cannot be activated, fragmented or unfolded using a simple capillary-skimmer interface.
Mass spectrometers that employ an ion funnel interface to couple the atmospheric pressure ionization region with the high vacuum region do not have a capillary-skimmer interface. Instead, the capillary is directly connected to a sub-atmospheric pressure region of the vacuum chamber containing the ion funnel apparatus. Here the capillary could be inline or orthogonal to the ion funnel axis. When the capillary is orthogonal to the ion funnel axis, an ion deflector plate is used to direct ions into the ion funnel. For a capillary-ion funnel interface it is even more difficult to achieve ion activation due to the high pressure at which ion funnels are operated as well as the mechanical design.
Therefore, there is a need for providing improved in-source ion activation, unfolding, and fragmentation in a high-pressure region of a mass spectrometer or other analytical device such as a stand-alone ion mobility spectrometer. There is also a need for providing improved desolvation and declustering of analyte ions prior to mass spectrometry analysis.